The present invention relates to a magnetic recording medium suitable for high-density recording and also to a magnetic storage device with said magnetic recording medium.
The magnetic recording system is divided into two types, that is, longitudinal recording and perpendicular recording, with the former being widely prevalent. The longitudinal magnetic recording system carries out magnetic recording by forming recording bits by the magnetic field generated by the magnetic head in such a way that the N-pole of one bit butts against the N-pole of its adjacent bit and the S-pole of one bit butts against the S-pole of its adjacent bit, the recording bits being arranged parallel to the plane of the magnetic recording medium. For this recording system to have a high recording density and to generate a high reproducing output, it is essential to reduce the effect of demagnetizing field on the recorded bits. To this end, attempts are being made to reduce the thickness of the magnetic film and to increase the coercive force in the magnetic film.
The perpendicular recording system performs magnetic recording in the following way. Recording bits are formed by the magnetic field of the magnetic head in the direction perpendicular to the film plane of the magnetic recording medium having the perpendicular magnetizing anisotropy, with adjacent bits being magnetized in the anti-parallel direction. Thus, the magnetic pole of one bit has a polarity opposite to that of its adjacent bit. As a result, the magnetic moments of adjacent bits attract each other. This stabilizes magnetization for recording and increases the coercive force, thereby contributing to high-density recording.
In both recording systems, an increase in coercive force is an important factor to improve the recording density. One of the factors to determine coercive force is magnetocrystalline anisotropy energy. This is a measure to indicate the ease with which the magnetic moment in magnetic crystal grains is oriented in a specific crystalline direction. The greater the value, the easier the orientation. For example, in the case of Co crystal grains, the magnetic moment easily orients in the direction of the c axis of the hexagonal closed-pack crystal lattice. (This is the axis of easy magnetization.) The magnetocrystalline anisotropy energy (or the magnetic anisotropy constant) Ku is 4.6xc3x97106 erg/cm3.
The energy to orient the magnetic moment in crystal grain in the direction of axis of easy magnetization is given by KuV, where V is the volume of crystal grain. On the other hand, the magnetic moment fluctuates due to thermal vibration. The energy of thermal vibration is given by kBT, where kB is Boltzmann constant and T is an absolute temperature.
The behavior of the magnetic moment varies depending on the relative magnitude of kBT and KuV. If kBT less than  less than KuV, the magnetic anisotropy energy is sufficiently large and hence the magnetic moment orients approximately in the direction of the c axis of crystal grain. If kBT greater than  greater than KuV, the energy of thermal vibration is larger than the magnetic anisotropy energy and hence the magnetic moment continues thermal vibration (super paramagnetic state). This thermal vibration causes the inversion of magnetic moment to take place with a certain probability per unit time. For example, the energy of thermal vibration required for the inversion of magnetic moment to take place with a probability of 1/e per second is 25 kBT. If this inversion takes place, the coercive force decreases as time lapses along with the probability, resulting in a decrease in recording density. Therefore, the recording medium should at least meet the condition of 25 kBT less than  less than KuV.
In the meantime, among the related art media for high-density magnetic recording is magnetic film of Co81Cr15Ta4 alloy. (See IEEE Transaction of Magnetics, vol. 34, No. 4 (July 1998), pp. 1558-1560, as the first U.S. literature.) This magnetic recording medium has a magnetic anisotropy energy Ku of 1.3xc3x97106 erg per cm3 at about 300 K (absolute temperature T).
The above-mentioned medium is characterized by a magnetic grain size of about 15 nm (on average) and a film thickness of about 20 nm. The magnetic anisotropy energy possessed by a single magnetic crystal grain is KuV=4.6xc3x9710xe2x88x9212 erg. On the other hand, the energy of thermal vibration at room temperature (300 K) is kBT=4.1xc3x9710xe2x88x9214 erg. Thus, KuV greater than  greater than 25 kBT. In other words, under the present condition of crystal grain size, the magnetic anisotropy energy is much larger than the energy of thermal vibration, and hence the magnetic moment is fixed in the direction of axis of easy magnetization and this leads to a sufficiently large coercive force.
For both recording systems to have an increased recording density, it is important not only to increase the reproduction output for high-density recording but also to reduce the noise of the medium. The noise of the medium in a state of high-density recording results from the zigzag magnetic domain wall in the transition region of the recording bit. The greater the fluctuation of the magnetic domain wall, the greater the noise. Thus, common practice to decrease noise is to reduce the particle size of the magnetic crystal grains constituting the magnetic recording medium, thereby to reduce the fluctuation of the magnetic domain wall in the transition region.
The related art recording density (as experimental data) is 10 Gbit per square inch. (See The 7th MMM-Intermag Joint Conference (January 1998), Session ZA papers.) This recording density corresponds to a linear recording density of about 400 kFCI (Flux Change per Inch, or magnetization reversal number per inch), with the bit length being about 60 nm, assuming that the ratio of bit length to track width is about 20:1, which is common.
The thin-film medium for longitudinal magnetic recording now in use has a crystal grain diameter of about 15 nm. This implies that only four crystal grains arranged in the bit direction currently constitute a bit which is 60 nm long. This results in a large zigzag magnetic domain wall in the transition region. In other words, the magnetic domain wall fluctuates so greatly as to give rise to a problem in noise.
If the related art medium described in the first literature given above is designed such that the crystal grain size is 10 nm and the film thickness is 10 nm according to the related art technique so as to raise recording density and reduce noise, then the magnetic anisotropy energy of the crystal grains will be KuV=1.2xc3x9710xe2x88x9212 erg. This value is about one-forth of that possessed by the related art magnetic crystal grain before design. However, the relation of KuV greater than 25 kBT is satisfied.
Unfortunately, simply reducing the crystal grain size and film thickness as mentioned above results in a medium which has a low coercive force at the higher operating temperature range (as mentioned later), and this medium does not produce sufficient high reproducing output. In other words, as the temperature of crystal grains rises by 50 K, reaching 350 K, the energy of thermal vibration increases to kBT=4.8xc3x9710xe2x88x9214 erg. On the other hand, the magnetic anisotropy energy usually decreases with increasing temperature, and it disappears at the Curie point. In the case of Co81Cr15Ta4 alloy described in the first U.S. literature mentioned above, Ku=1.3xc3x97106 erg/cc at T=300 K, while Ku=1.0xc3x97106 erg/cc at T=350 K. In other words, a temperature rise of 50 K causes Ku to decrease by 20% or more.
Consequently, the magnetic anisotropy energy of a single crystal grain at T=350 K is KuV=7.9xc3x9710xe2x88x9213 erg, and the relationship between the energy of thermal vibration and the magnetic anisotropy energy becomes KuV less than 25 kBT. The result is that the magnetic moment in a crystal grain is hardly fixed in the direction of axis of easy magnetization, and this in turn leads to a decreased coercive force and an unstable state of recording magnetization.
The present invention contributes to eliminate the above-mentioned disadvantages involved in the related art technology. It is an object of the present invention to provide a magnetic recording medium which has a high level of magnetic anisotropy energy, retains stably the state of recording magnetization within the operating temperature range, and is suitable for high-density recording with a low noise level. It is another object of the present invention to provide a magnetic storage device with said magnetic recording medium.
The above-mentioned object of the present invention is achieved by a magnetic recording medium which is characterized in that the magnetic anisotropy energy at 300 K is Kuxe2x89xa73.6xc3x97106 erg/cc and the average particle diameter (d) of magnetic crystal grains is 5 nm less than d less than 12 nm. (xe2x80x9cdxe2x80x9d is defined as the diameter of a circle having the same area as that of a magnetic crystal grain in the direction of the plane of the film.) The magnetic recording medium like this can be realized by the steps of forming an underlying film for orientation control, forming thereon an underlying film for lattice alignment, forming thereon a film of Coxe2x80x94Cr alloy magnetic body containing an added element, and performing heat treatment, thereby diffusing the added element into the crystal grain boundary. (This process will be explained later in more detail.)
The diameter of magnetic crystal grain was established as mentioned above for the following reason. As the grain size increases, the fluctuation of the zigzag magnetic domain wall in the transition region increases due to the crystal size distribution and the crystal grain arrangement distribution, thus increasing medium noise resulting from transition noise. By contrast, as the grain size decreases, the volume of crystal grain decreases and hence the magnetic anisotropy energy decreases. The optimum range was established in consideration of these two points.
In the case where the linear recording density is 400 kFCI (as mentioned above) and the bit length is about 60 nm, the number of crystal grains constituting the bit in the lengthwise direction should be at least 5. Four crystal grains are insufficient to eliminate noise. (This has been found from the study of noise.) The foregoing leads one to conclude that the average particle diameter of magnetic crystal grains constituting the magnetic recording film should be smaller than 12 nm. On the other hand, if the particle diameter of magnetic crystal grains is 5 nm or smaller, the volume of each crystal grain is excessively small. The result is that the energy of thermal vibration of magnetic moment is larger than the magnetic anisotropy energy of magnetic crystal grains to keep their magnetic moment in the direction of axis of easy magnetization. Thus, the magnetic moment cannot stably orient in the direction of axis of easy magnetization, and the magnetic crystal grains exhibit the properties of super paramagnetism. For this reason, the average particle diameter (d) of magnetic crystal grains should be within the range of 5 nm less than d less than 12 nm.
The above-mentioned magnetic anisotropy energy Ku was derived from the following view point. For the state of recording magnetization to remain stable regardless of temperature change, it is necessary that the ratio of KuV/kBT (where KuV is the magnetic anisotropy energy of magnetic crystal grains and kBT is the energy of thermal vibration of magnetic crystal grains) should have a sufficiently large value within the operating temperature range of the magnetic storage device or the magnetic recording/reproducing apparatus.
The relation between the magnetic moment energy KuV and the energy of thermal vibration kBT was studied from the view point of the stability of reproducing output with time. (IEEE Transaction of Magnetics, vol. 33, No. 5 (September 1997), pp. 3028-3030, as the third U.S. literature.) The authors of the literature measured the output of reproducing signals from the head immediately after bit recording and after standing for 96 hours. They found that the output decreased by only 4% after standing 96 hours if the KuV/kBT is about 85, whereas the output decreased by 10% or more after standing 96 hours if the KuV/kBT is about 55.
A decrease in output by 4% after standing 96 hours is desirable for the recording/reproducing characteristics of the magnetic storage device. Therefore, for the recording magnetization to remain stable in the operating temperature range of the magnetic storage device, it is necessary that the condition of KuV/kBT greater than 85 should be satisfied in the neighborhood of T=350 K which is the upper limit of the operating temperature. Also, in order to realize the high-density recording, it is necessary that the average particle diameter of crystal grains should be in the range of 5 nm less than d less than 12 nm. It follows from this crystal grain size that the magnetic anisotropy energy of magnetic crystal grains which meets the condition of KuV/kBT greater than 85 at T=350 K should be Ku (T=350 K)xe2x89xa73.0xc3x97106 erg/cc.
The magnetic anisotropy energy is the main cause of the coercive force of the recording medium. Therefore, when the magnetic anisotropy greatly varies within the operating temperature range of the magnetic storage device, the temperature dependence of coercive force also increases accordingly. Consequently, it is desirable to keep at about 10% the change of coercive force within the temperature range from T=300 K to T=350 K. It follows from this that the change with temperature in magnetic anisotropic energy should be such that Ku(T=350 K)/Ku(T=300 K)xe2x89xa70.85. This leads one to the conclusion that the magnetic anisotropic energy at T=300 K should be such that Ku(300 K)xe2x89xa73.6xc3x97106 erg/cc and [KuV/kBT] (T=350 K)/[KuV/kBT] (T=300 K)xe2x89xa70.73.
As mentioned above, the magnetic recording medium for high-density recording should be able to keep the state of microcrystalline grains and to stably retain the state of recording magnetization. To this end, it is necessary to make the magnetic recording film (layer) from a magnetic material which has a greater magnetic anisotropy energy than the related art one, and it is also necessary to minimize the change with temperature in magnetic anisotropy energy within the operating temperature range of the magnetic storage device.
Incidentally, in order to reduce noise resulting from the zigzag magnetic domain wall in the magnetization transition region of the medium, it is desirable to reduce the length and width of the magnetizing transition region between one recording bit and its adjacent bit.
The length and width of the magnetizing transition region of a medium is usually proportional to the product of xe2x80x9ctxe2x80x9d (the thickness of the magnetic recording film constituting the medium) and xe2x80x9cBrxe2x80x9d (the residual magnetic flux density of the magnetic recording film of the medium). Therefore, the smaller the value of Brxc2x7t, the lower the noise level and the better the S/N of the medium for high-density recording. On the other hand, a decreased value of Brxc2x7t leads to a decrease in magnetic flux leaking from the recording bit, which reduces the output of the reproducing head.
For this reason, the value of Brxc2x7t should be in the range of 30 Gaussxc2x7xcexcm less than Brxc2x7t less than 80 Gaussxc2x7xcexcm, so as to keep the medium S/N high and to prevent the decrease of output in the case of high-density recording.
A mention is made below of the material from which the recording medium is made. The magnetic anisotropy energy of a magnetic alloy is greatly affected by the combination and composition of elements constituting the magnetic material. In the case of an ordered alloy, it is greatly affected by whether the ordered alloy is in the ordered state or non-ordered state. In general, if Co is incorporated with a noble metal (such as Pt) or a rare earth metal element (such as Sm), the resulting alloy increases in magnetic anisotropy. If Co is incorporated with a non-magnetic element (such as Cr), the resulting alloy decreases in saturation magnetization and also magnetic anisotropy accordingly. As the amount of non-magnetic element added increases, the Curie point decreases, with the result that the magnetic anisotropy energy changes more with temperature in the neighborhood of room temperature.
The magnetic recording medium is a polycrystalline thin film composed of magnetic crystal grains. The magnetic properties of each crystal grain are responsible for the macroscopic magnetic characteristics of the thin film as a whole. In other words, if individual magnetic crystal grains have a large magnetic anisotropy energy, have a Curie point which is sufficiently higher compared with the operating temperature range of the magnetic storage device, and changes less with temperature in magnetic anisotropy energy in the operating temperature range, then the medium film would have macroscopic magnetic characteristics similar to them.
Thus, it is a good practice to increase the ratio of ferromagnetic element, noble metal, and rare earth element in individual magnetic crystal grains and to decrease the ratio of non-magnetic added element in them. However, simply increasing the ratio of magnetic elements in the magnetic crystal grains causes exchange interaction to act on between crystal grains. This increases noise resulting from the zigzag magnetic domain wall in the transition region between recording bits. This is not desirable from the view point of the recording/reproducing characteristics of the medium. Thus, it is necessary to non-magnetize the grain boundary so as to eliminate exchange interaction between crystal grains.
As mentioned above, there is a contradiction between the magnetic characteristics required of crystal grains proper and the magnetic characteristics required of the grain boundary.
In order to realize a medium which satisfies both characteristics, the film is formed from a Coxe2x80x94Cr alloy magnetic material by a process which employs a higher sputtering energy than the related art sputtering process and the resulting film undergoes heat treatment (as mentioned later in detail). This process permits the added elements to diffuse (for segregation) from the inside of magnetic crystal grains to the crystal grain boundary. In this way it is possible to realize the magnetic recording medium which meets the above-mentioned requirements.